1. Field of the Invention
The present invention relates to a direct current (DC) converter; in particular, to a current mode buck-boost converter.
2. Description of Related Art
The direct current converter is an important component for operation of electronic products. FIG. 1 shows a circuit diagram of a traditional H bridge converter. The H bridge converter is commonly used in existing buck-boost converters, and the H bridge converter comprises an inductor L and four switches SW1˜SW4. The switch SW1 is coupled between the inductor L and the input voltage VBAT. The switch SW2 and the switch SW3 are coupled between two terminals of the inductor L and the grounding terminal GND. The switch SW4 is coupled between a terminal of the inductor L and the output terminal. The output terminal is coupled with an output capacitor COUT. The H bridge converter is a DC to DC converter for converting the input voltage VBAT to the output voltage VOUT. The mentioned switches SW1˜SW4 can be embodied by power transistors and the input voltage VBAT can be provided by a battery.
The charging path and the discharging path are shown in FIG. 1. During the charging process, the charging path is composed of turned-on switches SW1, SW3 for charging the inductor L. During the discharging process, the discharging path is composed of turned-on switches SW2, SW4, and the electric energy stored in the inductor L would be transmitted to the output terminal to generate the output voltage VOUT. According to the inductor volt-second balance theorem, the relations between the average output current ILoad and the current of the inductor IL(avg.) can be derived as IL(avg.)=ILoad/(1−D), wherein the D is the duty-cycle. When VBAT=VOUT, D is 50%, the current of the inductor IL(avg.) are twice of the output current ILoad. The waveform of the current IL of the inductor L is shown in FIG. 2.
As mentioned above, the H bridge converter switches (turns on or turns off) the four switches in each cycle, and the switch process would lead to power consumption. When the duty-cycle is 50%, the current of the inductor is twice of the current of the loading, such that the power consumption would be four times of the power consumption of a boost converter or a buck converter.